Oxygen and nitrogen are among the most widely used chemicals in the world, the annual consumption of each gas amounting to in excess of 20 million tons in the United States. Most of this oxygen is used in the steel industry and related metals manufacturing processes. Oxygen-enriched air has also found significant uses, including treatment of waste water, non-ferrous smelting, glass production, medical applications, and other chemical oxidation processes. In addition, there is a great potential market for oxygen-enriched air in the synthetic fuels industry. Nitrogen and nitrogen-enriched air are useful primarily for inert blanketing atmospheres and for refrigeration.
More than 99% of all oxygen and nitrogen is currently produced by cryogenic fractionation, or a process involving lowering the temperature of air sufficiently (to about -215.degree. C.) to liquefy it and then using a multistage distillation process to produce pure oxygen and pure nitrogen. A major drawback of such cryogenic processes is that they require a great deal of energy and consequently are very expensive.
An alternate method that has been used for producing oxygen-enriched air is a process called pressure-swing adsorption (PSA). In this process, so-called molecular-sieve zeolites preferentially adsorb nitrogen from air, leaving behind most of the oxygen and other components (chiefly argon). Numerous PSA processes and variations have been patented. See, for example, U.S. Pat. Nos. 2,944,627, 3,142,547, 3,237,377, 3,280,536, 3,430,418, 3,564,816 and 3,636,679. All of these processes make use of materials that selectively adsorb nitrogen, rather than oxygen, argon or other gases that may be present in a feed stream. By reducing the pressure, the adsorbed gas can be desorbed, which regenerates the adsorbent. PSA processes thus involve regular cycles of adsorption and desorption to effect gas separations.
In the case of nitrogen separation from air, nitrogen adsorption generally takes place at an elevated pressure (typically 2 to 5 atmospheres) and desorption occurs at or below about atmospheric pressure. Under optimum conditions, sufficient nitrogen can be removed from air to produce up to 96% oxygen in the nitrogen-depleted feed stream. Higher oxygen contents cannot be achieved because oxygen and argon (the primary "contaminant" gas present in air) have nearly identical adsorption properties and thus are not separated. Since air contains approximately 78.1% nitrogen, 20.9% oxygen, and 0.9% argon, removal of all of the nitrogen and equal percentages of the oxygen and argon results in production of a gas containing about 96% oxygen and 4% argon. Under typical operating conditions, the product gas contains about 90% oxygen, 6% nitrogen, and 4% argon. To avoid contamination of the zeolite, water and carbon dioxide must be removed from air prior to the PSA process. An additional result of removing nitrogen, rather than oxgyen, from air is the presence in the product gas of other contaminant gases such as hydrocarbons, inert gases and oxides of sulfur and nitrogen that were present in the feed stream. PSA is economically competitive with cryogenic production of oxygen only in plant sizes up to perhaps 40 tons/day. Large-scale (100-3,000 tons/day) plants currently all use the cryogenic process.
It was observed by Tsumaki over 40 years ago in Bull. Chem. Soc. Japan 13 (1938) 252 that synthetic chelate-type compounds reversibly bind oxygen in the solid state. Subsequently, many researchers have investigated different chelate-type compounds in attempts to discover compounds that could be used to produce oxygen-enriched air. See, for example, the recent review by Jones, Summerville and Basolo in Chem. Reviews 79 (1979) 139. The most promising compound, commonly called fluomine,* has been studied for 35 years by the Air Force for potential use in providing breathing oxygen for crews of military aircraft. This compound is used to selectively bind oxygen at about 40.degree. C. and 400 mmHg oxygen partial pressure, and releases oxygen at about 110.degree. C. and 90 mmHg oxygen partial pressure. Fluomine is active in binding oxygen only in the solid state, and its activity is highly dependent on crystal structure. Furthermore, its useful operating lifetime is less than 10 days due to degradation of the fluomine at the elevated temperatures and pressures required for operation. FNT * N,N'-bis(3-fluorosalicylidene)ethylenediaminecobalt(II), described by Wilmarth, Aranoff and Calvin in J. Amer. Chem. Soc. 68 (1946) 2263, and by Adduci in Chem. Tech. 6 (1976) 575.
It is therefore a principal object of this invention to provide a commercially feasible non-PSA and non-cryogenic process for the separation of oxygen from atmospheric air or other oxygen-containing gaseous streams. It is another principal object of this invention to provide a commercially feasible non-PSA and non-cryogenic process for the separation of oxygen and nitrogen from atmospheric air.
These and other objects are accomplished by the method and apparatus of the present invention which is summarized and particularly described below.